Unary "~" performs bitwise negation, i.e., 1ís complement. For
example, "0666 & ~027" is 0640. (See also "Integer Arithmetic"
and "Bitwise String Operators".) Note that the width of the
result is platform-dependent: ~0 is 32 bits wide on a 32-bit
platform, but 64 bits wide on a 64-bit platform, so if you are
expecting a certain bit width, remember to use the & operator to
mask off the excess bits.